Lee Kuan Yew and fellow former prime minister Goh Chok Tong said in a joint resignation statement that the "time has come for a younger generation".

The 87-year old Mr Lee was prime minister from 1959 to 1990, after which Mr Goh took over until 2004.

Analysis

By Rachel HarveyBBC South East Asia Correspondent

Lee Kuan Yew has designed, driven, and dominated Singapore's development for over 50 years.

But now, aged 87, he says it's time to step down. He will give up his post as Minister Mentor, a cabinet advisory role specifically established for him in 2004.

The move comes after an election in which the opposition mounted their most effective challenge since independence. Mr Lee, under whose leadership, freedoms and rights were curtailed in return for a promise of security and prosperity, described the vote as a watershed.

"The time has come for a younger generation to carry Singapore forward in a more difficult and complex situation," he said.

The next government will be led, like the last one, by Mr Lee's son. The Patriarch's retirement is, unquestionably, a key moment in Singapore's political history. But the dynasty is secure.

Mr Lee had been known as minister mentor, while Mr Goh was senior minister since 2004. Both won parliament seats in the city-state's latest general election on 7 May.